1. Field of the Invention
The present invention relates to an optical system for forming an image on a color image sensing device such as a charge coupled device (CCD) in an image sensing apparatus for electrically capturing image information by the color image sensing device.
2. Description of the Prior Art
When image information is captured by an image sensing device such as a CCD in which pixels are uniformly spaced, for a subject of a high frequency, sampling of the uniform pixel interval causes image noise, called xe2x80x9caliasing noisexe2x80x9d, etc., of a low frequency, which is different from the frequency of the subject. To prevent problems caused thereby, it has conventionally been known to provide a crystal-made optical low-pass filter within the optical system of the image sensing apparatus so that the high-frequency components of the subject do not reach the image sensing device.
Recently, in order to reduce the size and cost of the image sensing apparatus, a single-plate-type CCD has been frequently used in which color filters of red (R), green (G) and blue (B) are alternately disposed as pixels of the CCD. In that case, it is common practice to increase the image performance by disposing pixels of G which appeal to the viewer""s eyes the most, in a greater number than pixels of R and B. FIG. 2 shows an example of color pixel arrangement of the CCD. This arrangement is called a checkered pattern.
However, in the above-described CCD, the xe2x80x9caliasing noisexe2x80x9d is conspicuous for R and B with large pixel intervals since the numbers of pixels of R and B are small, and this appears as a defect called xe2x80x9ccolor moirexe2x80x9d which is a colored noise. For such CCDs, the above-mentioned optical low-pass filter is used, particularly, for making the xe2x80x9ccolor moirexe2x80x9d inconspicuous.
A characteristic of the conventional optical system including the optical low-pass filter will be described. It is assumed that the pixel pitch p of the CCD used here is 0.005 mm. This value applies to the present embodiment. In the conventional optical system for the image sensing apparatus, in order to restrain the generation of xe2x80x9ccolor moirexe2x80x9d, an optical low-pass filter in which the modulation transfer function (MTF) is 0 approximately at the Nyquist frequency xcexdn is disposed between the lens and the CCD. Here, xcexdn=1/(2p)=100 mmxe2x88x921.
Therefore, the MTF characteristic of the optical system including the optical low-pass filter is as shown in FIG. 3 for all of R, G and B. In the figure, the lateral axis represents the spatial frequency and the longitudinal axis represents the MTF. The MTF here, which is 0 at the Nyquist frequency xcexdn as shown in FIG. 3, is appropriate as the MTF for R and B images which are factors of the generation of the xe2x80x9ccolor moirexe2x80x9d; however, in the G images having a smaller pixel pitch than the R and B images, the MTF is so low that the MTF of the final image is greatly decreased.
However, when the optical low-pass filter is used, although needless high frequencies can be cut since the xe2x80x9ccolor moirexe2x80x9d is restrained, images of low frequency components are also deteriorated and, particularly, in the above-described single-plate-type CCD, high frequency components which are not unnecessary are also cut for the pixels of G. In addition, since the optical low-pass filter is very expensive, the cost of the image sensing apparatus increases.
An object of the present invention is to provide an image sensing apparatus in which the above-mentioned problems are solved, the generation of the so-called xe2x80x9ccolor moirexe2x80x9d can be restrained without the use of the optical low-pass filter, image quality is obtained which is higher than that of the conventional apparatus using the optical low-pass filter, and cost reduction is achieved.
To achieve the above-mentioned object, according to one aspect of the present invention, an image sensing apparatus is provided with: an optical lens for imaging light from an illuminated subject; and a color image sensing device for photoelectrically converting the imaged light for each color, the following conditions are fulfilled:
1.5*p*f# less than |LBgxe2x88x92LBe| less than 5.0*p*f#
1.5*p*f# less than |LBcxe2x88x92LBe less than 5.0*p*f#
where p is a predetermined pitch of a photoelectrically converting pixel of the color image sensing device, LBe is a paraxial back focal length of the e-line (546.07 nm in wavelength), LBg is a paraxial back focal length of the g-line (435.84 nm in wavelength), LBc is a paraxial back focal length of the C-line (656.28 nm in wavelength), and f# is an F-number of the optical lens.
Moreover, the color image sensing device is a charge coupled device having photoelectrically converting pixels of primary colors of red, green and blue. Further, the amount of exposure to the color image sensing device may be controlled by varying illumination intensity for illuminating the subject. Alternatively, the amount of exposure to the color image sensing device may be controlled by varying illumination time for illuminating the subject. Moreover, the amount of exposure to the color image sensing device may be controlled by providing a filter for changing transmittance of the light from the subject on an optical path between the subject and the color image sensing device.